1. Field of the Invention
The present invention relates to an antenna for use in a portable radio apparatus and, more particularly, to an antenna which is to be incorporated in the housing of a radio apparatus or in a peripheral device to the radio apparatus.
2. Description of the Prior Art
Portable radio apparatuses of various types, such as portable radio receivers and pagers, are commercially available. They are used in great numbers because they are small, light and useful. They have an antenna to receive radio waves. In most cases, the antenna is provided in the housing of the apparatus or in a peripheral device to the apparatus.
The recent advancement in the integrated circuit technology has provided miniaturized components of radio-circuit components which consume but a little power. Additionally, small, high-performance and large-capacity dry cells and rechargeable batteries for use in portable radio apparatuses have come into practical use. However, antennas for use in portable radio apparatuses have yet to be miniaturized. This is because the power an antenna can output is proportional to the wave-receiving area of the antenna and the length of the antenna. It should be noted that the antenna length is closely related to the lengths of radio waves to detect.
Among portable radio apparatuses hitherto developed is a watch-shaped one which comprises a case and a band. If it is an AM radio, it has a bar antenna provided within the case, for receiving MF (Middle-Frequency) radio waves. If it is an FM radio or a pager, it has a loop-type band antenna incorporated in the band, for receiving FM (Frequency-Modulated) radio waves. A portable FM radio receiver and a pager, i.e., two other types of portable radio apparatus, have a cord-type antenna which functions as an earphone, as well.
Conventional antennas, such as a bar antenna, a cord-type antenna and a loop-type band antenna, for use in portable radio apparatuses, are disadvantageous in the following respects.
(a) The bar antenna or the like to be set within the case of a watch-shaped radio apparatus cannot perform a desired function if used in combination with a pager, a mobile telephone or a personal digital assistance (PDA) having a radio receiver/transmitter, which needs to receive high-frequency radio waves of hundreds of megahertzes to several gigahertzes. Further, in order to accommodate the bar antenna or the like, the case must be made of electrically conductive material such as metal.
(b) The cord-type antenna for use in a portable FM radio receiver, which functions also as an earphone, has to be connected to or wrapped around the receiver when it is used.
(c) The loop-type band antenna has a complex structure, and the manufacturing cost of its antenna section is high. This is because a loop must be formed when the antenna is connected to the buckle of the wrist band. Since the antenna is wrapped around the wrist, the diameter of the loop changes with the size of the wrist, inevitably changing the antenna length. To maintain the characteristics of the antenna, an adjusting circuit must be used to compensate for the change in the antenna length.
(d) Even if a metal conductor is bonded to the band of the watch-shaped radio apparatus, the characteristics of the antenna remain unstable. This is because the size of the antenna is limited and also because the conductor or the wrist, which is also a conductor, extends through the antenna loop. As a consequence, the antenna cannot have a sensitivity as high as desired and cannot receive or transmit radio waves reliably.
(e) Generally, the ratio of the radiation resistance to the input resistance is small in the loop antenna. Further, the loop antenna cannot be used unless the input reactance is canceled out. It has be used at an extremely low efficiency.